1. Field of Endeavor
The present invention relates to an axial turbine and a method for discharging a flow from an axial turbine, wherein the turbine is a steam turbine.
In particular the present invention refers to a design for the guide vane of the last stage and/or for the stages upstream and/or downstream of extraction slits, which favorably influences the work extraction and the flow in the exhaust diffuser.
2. Brief Description of the Related Art
Steam turbines are known to have a cylinder including a plurality of expansion stages made of stator/rotor blades.
The stator blades of each stage are all identical, arranged with identical geometrical configuration (i.e., they have the same stagger angle, etc) and define guide vanes that are identical; in the same way, also the rotor blades of each stage are all identical, are arranged with the same geometrical configuration (i.e., they have the same stagger angle, etc) and define paths that are identical.
Downstream of the expansion stages, steam turbines have an exhaust diffuser that collects the steam coming from the expansion stages and typically (for power production plants) discharges it into a condenser.
The exhaust diffuser is made of an axial-symmetric portion that gathers the steam coming from the last expansion stage and feeds it to a non axial-symmetric collector, having an upper quasi-circular casing portion and a lower discharging casing portion with a rectangular opening connected to a condenser neck.
During operation, the steam passes through the expansion stages and delivers mechanical power to the rotor blades (and thus to a turbine shaft connected to the rotor blades).
Subsequently, the steam flowing out of the last expansion stage enters the exhaust diffuser, where it is collected and discharged into the condenser.
Nevertheless, as the expansion stages of the turbine are axial symmetric, whereas the collector of the exhaust diffuser is neither axial symmetric, nor extends in the same direction as the expansion stages, when passing through the exhaust diffuser the steam flow undergoes large circumferential distortions.
This causes the operating conditions of the steam in the last expansion stage (in particular in the circumferential direction) to be influenced, such that in the last expansion stage the operating conditions of the steam are not circumferentially uniform.
Moreover, flow circumferential distortions cause non-uniform mixing losses within the steam flow and differing pressure drops that can worsen the overall efficiency of the steam turbine.
The same drawbacks are caused by non-axial-symmetric portions of the turbine, defined, for example, by extraction slits; in this case the steam flowing through the stage upstream and downstream of an extraction slit is influenced by the extraction slit.